>>>READER-TO-READER QUESTIONS AND ANSWERS
cause the radio to lose the signature
signal from the tester transmitter.
Repeat the tests after doing any
needed repairs and then restore the
#3 Dog containment systems
(buried wire RF) operate on a variety
of frequencies, well below the broadcast band. (Many operate around 18
kHz, but the exact frequencies vary
from manufacturer to manufacturer,
and some offer selectable channels to
avoid interference with neighbors.)
This is classified as VLF, and VLF radiates up out of the ground very well.
A tunable field strength meter
would be a helpful tool, although very
few on the market tune down that far.
As far as breaks in the wire go,
usually the buried wire is a solid
conductor with relatively thin insulation. Shovel nicks, nicks from poor
backfill, etc., tend to cut the insulation
and it is just a matter of time before
the copper conductor inside turns to
green powder (or white power, in the
case of aluminum). In this case, an
earth gradient fault locator for buired
cables (like the Aqua-Tronics EG-3000;
www.Aquatronics.com) will help you
in locating the break.
A commercial field strength meter
and earth gradient locator will both set
you back quite a bit. If it is your own
system you are working on, it would
be more cost-effective for you to
re-bury the line, this time using a
stranded cable with a jacket rated for
direct burial. If you can't find such
cable in your area, you can always
bury type UF Romex (like you
would run to a lamp post), with the
three conductors tied together for
redundancy. Any splices needing to
94 December 2008
be buried should be coated in pitch or
tar, then enclosed in a couple of layers
of heat shrink tubing.
[#10081 - October 2008]
I am looking for an efficient,
physically very small, low parts count
circuit to provide a regulated 5V at up
to 10 mA output directly from a power
line (115 VAC) input.
Warning: Non-isolated power
supplies present a potentialy lethal
connection to the AC line and should
only be used by persons who
completely understand the hazards
and appropriate applications.
#1 Figure 1A: Very efficient, small
parts count, and dirty (hazardous).
Two capacitors, two diodes, and a
zener. The only power consumed by
the circuit is what the zener will be
sucking up with no load; 10 ma 5V
= 50 mw. The capacitor will return
unused power to the line.
Figure 1B. Very efficient, more
parts count, and not quite so dirty.
Three capacitors, two diodes, and a
zener. One capacitor improves safety.
Keep in mind these circuits
without the zener is a voltage doubler,
so you will be dealing with 230 volts
and our zener sees only 1/2 wave
rectification. Double voltage doubles
1/2 wave rectification for total amperage. This circuit is a current source, so
treat it with a lot more respect than
you would a transformer five volts
circuit. For the diodes, as long as this
circuit is fully assembled when tested
1N4001s are adequate. Otherwise,
500 volt diodes should be used; 115
AC volts doubled is nearly 330 volts
peak; 1.414 115 2 = 325 volts.
Basic formula Xc = 1 / (2pi F C),
Therefore C = 1 / 2pi F Xc. Xc is the
desired impedence to the frequency
(F = 60 Hz). R = E / I , R = 115 / .01 =
11,500 ohms, Xc = R. C = 1 / ( 6. 28 *
60 11,500) = 2.3 μf. If you do not
mind a little less amperage, use a 2 μF
cap for Figure 1A, or put a . 5 μF cap in
parallel with the 2 μF cap. Two 5 μF
caps for 1B.
For parts, you could take a bad
compact florescent bulb base apart.
Inside are 1N4007 diodes (1,000V
1A), 200 volt capacitors at 10 μF for
60w and 20 μF for 100 watt equivalent bulbs. The base case is plastic; the
groove around the base is a four snap
tab fitting. With a little persistence and
a small screwdriver, it will come apart.
Alonzo E. Fuller
Sweet Home, OR
#2 There was no requirement in
the question that the output be
isolated from the line, so I designed
this capacitive coupled supply (Figure
2.) The safety ground is connected to
neutral so there is no chance that
common will be hot. The TPS76350
has a maximum input rating of 10
volts; that is one reason for using an
8.2 volt zener at the input. The layout
(Figure 2A) is 1.15 inches by 1.35
inches, including two mounting holes.
All Mouser part numbers.
#3 Bryan, get your soldering iron
out and download Microchip App